The present invention relates to medical electrical leads and, in particular, to conductors for such leads.
Early cardiac pacemaker conductors were composed of numerous fine, stranded stainless steel wires. Marked improvement in both fracture rate and flexibility resulted when stainless steel conductors were wound into small coils with a hollow core. The hollow core of the coils also improved implantation since a stylet could be passed through the core during implantation to stiffen the lead. Corrosion resistance was significantly increased when stainless steel was replaced with more corrosion-resistant platinum-iridium and nickel alloy materials such as MP35N. Highly specialized conductors were used with such materials such as the use of multifilar wire coils (to avoid loss of electrical continuity in the event that one wire breaks) and drawn brazed strand (DBS) wire material (to provide a low electrical resistance in a wire of high fatigue strength). Multifilar coils can also be used in side-by-side or coaxial arrangements with insulation separating the conductors to provide individual conductors for the transmission of separate signals or stimulation pulses. However, it has been noted that polymeric materials (such as polyether urethanes) used for lead insulation can be adversely affected over long periods of implantation by metal ions from the nickel alloy conductors. Accordingly, it would be desirable to replace nickel alloy conductors like MP35N with other conductors which would not exhibit such a problem.
Of critical importance in this effort is to find a wire material that can be used in a multifilar coil wire geometry that will not fail under the mechanical stresses to which the lead will be subjected. The motions an implanted lead can experience are tension, twist and bending within the coil wire. Each of these produce either normal (tensile or compressive) or shear stresses which occur in all directions, but certain directions predominate depending on the modes of motion involved. If the magnitude of these stresses are too great with respect to the fatigue strength of the material, the structure will fail. Also of great importance is to find a wire material that will provide low electrical resistance and corrosion resistance. Titanium has been suggested as a lead conductor in U.S. Pat. No. 4,355,647 issued to Kallok et al. Titanium is known for its inertness to many corrosive environments. For this reason, titanium has also been mentioned in U.S. Pat. Nos. 5,040,544, 4,947,866 and 4,860,446 issued to Lessar et al as a suitable material for sputter coating onto nickel alloy lead conductors to reduce the release of harmful ions. However, it has not been clear which, if any, alloys of titanium could be used for lead conductors.
It is therefore an object of the invention to provide a medical electrical lead having a titanium alloy conductor material that will not promote the degradation of adjacent polymeric materials.
It is also an object of the invention to provide a medical electrical lead having a titanium alloy conductor material with good fatigue strength when used in a coil geometry.
It is also an object of the invention to provide a medical electrical lead having a titanium alloy conductor material with low electrical resistance.